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In footnotes or endnotes please cite AIP interviews like this:
Interview of Paul Epstein by John L. Heilbron on 1962 May 25,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
Part of the Archives for the History of Quantum Physics oral history collection, which includes tapes and transcripts of oral history interviews conducted with circa 100 atomic and quantum physicists. Subjects discuss their family backgrounds, how they became interested in physics, their educations, people who influenced them, their careers including social influences on the conditions of research, and the state of atomic, nuclear, and quantum physics during the period in which they worked. Discussions of scientific matters relate to work that was done between approximately 1900 and 1930, with an emphasis on the discovery and interpretations of quantum mechanics in the 1920s. Also prominently mentioned are: Niels Henrik David Bohr, Arthur Compton, Peter Josef William Debye, Carl Henry Eckart, Paul Ehrenfest, Albert Einstein, Peter Paul Ewald, Folsom, Kasterin, Felix Klein, Hendrik Anthony Kramers, Max Theodor Felix von Laue, Petr Nikolayevich Lebedev, Hendrik Antoon Lorentz, Robert Andrews Millikan, Walther Nernst, John William Nicholson, Max Planck, Adalbert Wojciech Rubinowicz, Erwin Schrödinger, Karl Schwarzschild, Arnold Sommerfeld, Johannes Stark, Timiriazev, Umov, Theodore von Kármán, Wagner, Hermann Weyl; California Institute of Technology, Rijksuniversiteit te Leiden, Universität Leipzig, University of Moscow, and Universität Munich.
We can begin, if you will, at the beginning of the outline with your own interest in physics.
It was early in life. I do not know how my interest in science arose, because as far back as I can remember, it was always there. My mother demonstrated me at the age of four and told them that I was going to be a mathematician. By the age of ten I was very definite that I was going to study physics.
And was there any particular reason to go to the gymnasium in Minsk?
Well, that was the only school that led up to the university, within a 100 miles. It was for no other reasons that I chose the University. I had no particular preference for it. I didn’t know what else to do. It was the hardest to get into so that was a stimulus too.
Were there other reasonable places to study physics at that time in Russia besides Moscow?
You see I have no idea. Being a provincial boy I didn’t know who the professors were there, and which professors were good and which not. There was a complete isolation from everything in the province. Books were very hard to come by, though I managed through a student friend of mine who studied in Moscow to get the names of some of the titles and with great difficulty I got them through the book seller. But it took a couple of months to get the books. And by the time of what you call my sophomore year I had studied integral calculus and knew more or less of the theoretical mechanics and so on. The books I acquired were Russian books. I was bilingual. I spoke German as well as Russian from my childhood. But there was a difficulty to get German books in Russia then. The bookselling business was not sufficiently organized for that.
Were there many opportunities for people to continue in the sciences at that time?
No. Especially for Jewish people, there were practically no opportunities. But that didn’t stop me because I had in mind to go abroad anyway — not to stay in Russia. That I went to Moscow and not to a western university was just to humor my parents. You see the university gave to the Jew great privileges — the privilege of living anywhere in Russia. Ordinarily we were restricted to a small fraction of it. So for that reason I went to Moscow University.
Was there already a very small quota on the Jews allowed in the University?
Two percent in Moscow, of the what, you call freshman class. Two percent of the students accepted. In the other universities it was three percent. Moscow was different. And in the Jewish Pale, in the gymnasium and the high school they had 10 percent, which was very low because the city was about 30 percent Jewish. This was also 10 percent of those who entered. Now of the originals, many, many dropped out during the course of the gymnasium, so that by the time you graduated there was perhaps 20 percent Jewish. Most of them managed to get into the university, but in one of the inferior, provincial ones.
Had Lebedev no influence in attracting you to Moscow?
Yes, we’ll come to that in time. I hadn’t heard the name of Lebedev until I was in my sophomore year in the University. He was the dominant figure in Russian physics you may say. Now he was not the dominant figure in any way in the administration or in the importance of the University. He was just a young professor. He was recognized internationally but not at home.
Did the government control the courses offered?
There was in Moscow an older physicist named (Umov) who gave the regular physics lecture course which I took, who had a high standing. He was by then a fairly old man, but he had gotten an honorary doctors’ degree from the University of Glasgow in the late 1870’s, so he was internationally known then, namely because he had independently of Poynting discovered the Poynting’s theorem in a more general way. That was why he was honored by a doctors' degree. But I had little to do with him. I didn’t think to say that I fortunately was from a pretty well-to-do family, so that the immediate bread-and-butter considerations were not important to me. Then, the physics course in Moscow was an undergraduate course. It was the best that I have ever seen, even in Europe or in America. In the general university, the physics courses are one year courses, but in Russia it was a two year’s course. So we had time to go into it much more thoroughly, especially the demonstrations. It was essentially a demonstration course, doing very magnificent experiments for the students. This stems from the great Russian physicist ( ), who was professor in Moscow for a long time. And I saw a lot of beautiful experiments which students here and in Germany never see. You know perhaps the book of Pohl on experimental physics. The Russian course had about twice as many experiments as Pohl. In addition to that course we had a parallel course in problems of physics, wrote at the blackboard and turned in papers and so on. That was given by an assistant professor, a Privatdozent. That was really the most interesting for me, because that is where I learned physics. This was an obligatory course for all students in the mathematics division, and also the chemists; so they could not make it very hard. But it was good enough.
Did it include Maxwell’s theory?
No, it did not. We were still in the old elastic theory. And I had to learn Maxwell on the side. But we had an adequate resume of kinetic theory, that was short, but correct. As for thermodynamics, we had the same disadvantage that the American students have, that in the lecture of physics we didn’t know yet enough mathematics so that differentials and so on couldn’t be treated. So we had thermodynamics in a very rudimentary fashion. That was the undergraduate course. Then for those students who wanted, there were no elective courses, but there were courses in theoretical physics which were given by a Privatdozent named Kasterin, who was an excellent mathematician. He had studied with Lorentz for a time and Lorentz had a high opinion of him. But he had not a wide outlook, he was a narrow specialist interested in the theory of dispersion primarily. I entered the University in 1901. The political conditions were rather unstable, and the University didn’t function very regularly. For one year it was completely closed, but always it was on the verge of being closed. So I didn’t get a complete course — in fact I took only two courses of this Kasterin. One in thermodynamics, called theory of heat, which was pretty simple thermodynamics and theory of heat conduction. The other was in acoustics. Besides I had two other advanced courses — they would be here graduate courses. One was theory of heat radiation by the same Zinger I mentioned. And here was a really interesting thing. In the theory of heat radiation, he finished up with Planck’s derivation of his law. But he didn’t mention the quantum at all — quantum of action. You got nothing but the straight forward classical derivation. In fact, in Planck’s original paper there are just two lines about the quantum, so that an inattentive reader would overlook it.
Then the problem had been solved in Russia?
Ja. Now you see I took that course in my sophomore year, and if I didn’t understand quite I could think, “Very well, I am not ready yet to understand.” So I didn’t realize that there were any problems. That was in 1902 or ‘03; anyway it was before Planck’s book came out, which was in 1906. Zinger was a really intelligent fellow.
So previous to that time the problems of the radiation theory, the difficulties of solving it, the Rayleigh problem and so on, didn’t excite many people?
Well, the difficulties were for the first time pointed out by Einstein in 1905. There he made the great discovery that there was such a thing as the quantum of action. He had also read Planck, and never realized that it came up. But that’s where the words that one must assume that there is a discontinuity, that radiation is emitted in quanta, are announced to the world for the first time.
Up to that time it completely escaped everyone?
It escaped us certainly, and I think it escaped the whole world, and maybe even Planck, I don’t know. The quantum became really a problem when I graduated. Well, I graduated actually in 1906 because I (???) class for a year. You see you could not graduate there when you wanted to. The course was four years and that was that. The other good advanced course was given by Timiriazev, and was called problems of modern physics. That was very interesting, but they were exclusively problems of experimental physics, so the quantum wasn’t mentioned, which was not surprising. But the relativity wasn’t mentioned either.
How did it happen that you, who were interested in mathematical physics, got your degree under Lebedev?
Well you see, when I entered the University my intention was to study mathematical physics. But, there was no mathematical physicist in Moscow, and none in Leningrad or other University. So that I naturally came in contact with Lebedev. I didn’t quite finish telling about the instruction that was given in Moscow. We had in general no laboratory course, except for a few students. There was a very inadequate laboratory. Umov had one and the other professor, Soholovich had one. Lebedev was not a professor when I entered, but he became so only a year later. He was still Privatdozent. Soholovich gave a laboratory course for 35 students, beginning with the sophomore year since they had to pass the first year’s examination. Now that included also those from the mathematics division and those from the chemistry and natural science division. All together there were some 300 students; only 35 of them could get into the laboratory. So it was certainly not a general course, but I did get into it. And there I came for the first time in contact with the personnel of physics. When you just listen to lectures you don’t know them. Lebedev had there a room. He was an assistant of Soholovich then… However, by that time it was well-known that Lebedev was a coming man. By the end of my laboratory course — that was in the second half of my sophomore year — I went to Lebedev and asked whether I could work under him, and he accepted me. He was not only a good physicist but a very impressive and strong personality.
He didn’t think much of theoretical physics, so I accepted this, and decided to become an experimental physicist. And I was an experimental physicist until I noticed that I hadn’t the stuff for it. I graduated then under Lebedev. You did not need a thesis because we couldn’t experiment — very few had access to the laboratories. But I wrote a sort of thesis about the electron. I reviewed all the literature relating to the electron, wrote a thick book about it and turned it in. But that was not research work. The research in the laboratory was on dielectric constant of gases, to work out a method which needed a very small amount of gas. He intended to measure the noble gases, the helium group, which hadn’t yet been measured in that time. Emanations were not yet known when he gave me that assignment, they were only discovered in the course of my work. But I worked out the method, and there was a short account of it in one of the Russian journals but that was all. I turned it over to another man however who made the actual measurements, and proved the method. By that time I was started to work on my own.
You see, the organization was such that when you graduated, you could not enroll any more in the University except in another section of it. But a few men who were promising were appointed to what is called “aspirant,” that, is what we call here graduate students. But the aspirants didn’t take any courses, it was only they were given a standing so that they had a right to come to the University. The Russians have great formalism and unless you have that pass, you are not let in. So the aspirant had the facility of the library, and if it was an experimental subject, then also the laboratory. So I continued to work in the laboratory. Since at that time I still worked experimentally, I had no access to the lectures — there was no graduate lecture teaching. So what I wanted, I had to learn from books and journals. But, we had a weekly colloquium what they call here the seminar. It was just for Lebedev’s group. I attended already as an undergraduate student, but I did not report in it. It was a general colloquium. As soon as I graduated I could then offer a report. And that was immensely instructive. It was the best colloquium that I ever attended.
Did it review theoretical papers as well, or did it limit itself to experimental work?
Ja, theoretical papers were given, but it had to be boiled down to the understanding of the experimentalists there. There was, apart from me, two elder men who were interested in theoretical physics. One was Esmarch. He published not much, but quite good papers. And the other was Timiriazev, the son of the botanist, a very famous man in Russia, also internationally known, who was fair. These two were the theoreticians, the others were experimentalists. I was myself an experimentalist, though, at home I studied some theory. We didn’t hear in the graduate year, or even in the colloquium, of quantum theory, or theory of relativity. Relativity was not in my colloquium because it was not invented in 1905. It became really generally known only in 1908 after Minkowski's paper. I was in Russia until 1910, and became myself Privatdozent. I was already assistant professor when I left. … Apart from this dielectric constant of gases, which took most of my time, I wrote two theoretical papers. One on the structure of the electron. Independently it came cut and then later Poincare and Lorentz commented on it.
Well, it was not very interesting. I found the pressure on the surface of the electron isn’t variable; even when it is accelerated it has constant pressure and the same pressure. It is what is called now a relativistic invariant. I reported it in the Russian Congress for the Advancement of Science in 1909, and there was a short report printed. And the other was inspired by (Wiechert,) who had worked on a theory of terrestrial magnetism — how you can get a magnetic field by rotation. I made a theory cut of that, which incidentally, was after (Wiechert’s) work was done, and it came out that he needn’t have bothered, that it was much too small, but he didn’t believe in theory anyway. He tested it experimentally. That came out when I was already in Munich; I turned it in at the time I left. But you are interested in quantum theory, and what was the point of view. Now I tell you, the point of view was negative. Those two specialists in theoretical physics, Esmarch and Timiriazev, were both against it. Esmarch was influenced by Jeans. Jeans wrote a book which is essentially a stupid book — he throws doubt on the experimental side of things. Maybe Rayleigh’s law is after all correct; the measurements may be wrong and therefore Rayleigh’s law is correct, which is obvious nonsense. But somehow Esmarch liked that. And Timiriazev on the other hand was entranced by a paper of J.J. Thomson who deduced Planck’s law without quantum. There was such a paper in I think late 1909. So that we had no incentives to believe in the quantum.
The theory of relativity just broke out. You see, those whom I mentioned, were the physicists of the University. In Moscow there were also other institutes, and there was (Eichenwald), who was you may say the only physicist who really worked in modern physics. Because the problems of Lebedev were classical. He had two main problems, one the pressure of radiation, which he followed through all kinds of light waves, sound waves, hydrodynamic and so on. And the other was micro-wave. In micro-waves Russia was really the leader of the world. You know that in 1896 Lebedev got down to millimeter waves. And we had, in that lecture course, demonstrations with 10 and 20 centimeter waves in the lecture room, with all the reflections and polarizations, and everything. Occasionally he was interested in other things. Lebedev was very fertile in problems, but he was not the right teacher for me.
You spent some time just after you graduated teaching at a Landwirtschaftliche Hochschule?
Ja. When I graduated I was appointed aspirant at the University, which gave me the right to continue to work in the laboratory there, and I was also at the same time appointed laboratory assistant in that agricultural institute. The professor there was Michelson, who was a good physicist, but of very precarious health. He had a pretty big teaching load, so that with poor health and many duties he couldn’t do much original work. But he was an interesting man. I was working in the University experimentally I only spent three nights a week there teaching the laboratory courses for students. But that was only for one year. The next year I did the same in the University, was a teaching assistant there, and I continued also after I was appointed Privatdozent. So that exhausts Moscow.
Yes. Shall we go to Munich? There must have been some great differences.
The differences were great. You see, Sommerfeld was a very pleasant and nice man, very approachable. And he liked to talk. That was his method of working. Every problem he helped, he helped by chewing it over with his surroundings, students or assistants. You asked why I went there. Well, because my main purpose was to learn mathematical technique, and Sommerfeld was the best man for that. Of course in my group in Moscow nobody knew very much about teaching of theoretical physics and who the good men were. But at that time Ehrenfest lived in Russia, and he came to that same Congress which I mentioned. Such a Congress for the Advancement of Science was held in Russia every four years, and this one was at the end of 1909.
So be came to Minsk, Russia, and I constituted myself his bodyguard for this meeting. I took him around everywhere. And he already was a thoroughly great mathematical physicist, the first time I saw him. So I asked him, and he said, “Go to Munich, by all means.” I liked Munich anyway, so I took his advice, and didn’t regret it. Then another asset was that Sommerfeld was in correspondence with every other theoretical physicist in Germany. He was himself a new man. He was originally a mathematician, then he was professor of applied mechanics in Clausthal and Aachen, and was appointed successor of Boltzmann in Munich because of his work on the electron. And in general his mental constitution was that his strength was in solving problems and not in finding problems. For that reason… the problems which he solved actually were in many cases his students’ problems. Now besides this intimate touch of his with Einstein, Planck, he was himself a Gottingen man and at Gottingen all the professors were his bosom friends. You were there just at the horse’s mouth, so to speak. He knew what everybody was working on long before the papers came out. So that from the backwoods I was transported into the very center of theoretical physics. But still the quantum theory held a very subordinate place there. At that time he was interested in relativity. His early physical work was all on Maxwell’s theory — applications in all directions. But as Minkowski was his close friend — Minkowski, Hilbert, and Sommerfeld were all from Konigsberg and knew each other in the gymnasium — Sommerfeld then concentrated on relativity. And when I came in the middle of the semester he invited me to come into his lectures on relativity.
The problem which he gave me was on electromagnetic waves. It was an easy problem which I finished in a few weeks, and then he gave me a harder problem, which was too unwieldy. I learned a lot with it, but then I found a problem that could serve just as well. It was the diffraction from a parabolic cylinder. It worked out, and I got my Munich Doctor’s degree on that. But the main problem is, what was the attitude [in Munich] toward the quantum then. Well, when I came  there wasn't much yet. Of course they were aware of such a thing’s existence which we were not [in Russia], but they were not particularly interested in it. His assistant was Debye. Sommerfeld never really became a physicist, in my opinion — he just stayed a mathematician, though a very brilliant one. Debye came from Aachen, and was originally an electrical engineer, and was assistant of Sommerfeld’s in Aachen already; he is an all around able man. In physics Sommerfeld deferred to Debye very much really. And Debye again got captivated by this paper of J.J. Thomson where Planck’s formula is derived without the quantum. So they did not see the necessity of the quantum until Sommerfeld — it must have been 1911 — needed a rest, though it was in the middle of the year, and for a few days he went to Einstein at Zurich. The idea of recreation was to him to talk the whole day physics with Einstein. And there he told Einstein that Debye was enchanted with the theory of J.J. Thomson. Neither of them read much physics, neither Sommerfeld nor Einstein; Einstein had too many ideas himself, and Sommerfeld professed that if you wanted to know, something about Planck’s theory you wrote a letter to Planck. So that actually for the first, time Einstein then, heard about that J.J. Thomson paper and told his assistant to get the book from the library. They had no library in the physics department in Zurich. When I was there I started a little library later. So they got that volume, and then they read it together. And Einstein laid his finger on the error. So J.J. Thomson was punctured. // But this did not disturb Sommerfeld.// You see one of the advantages of not being a physicist, Sommerfeld had no prejudices, one way or the other.
The quantum to the physicists was simply a very outlandish idea at that time, but to Sommerfeld on the other hand, why not. So that he and Debye a year later even made up a theory of emission based on the quantum theory. When Debye told me about it, I had to disappoint him, because it was already two times published. You see, I was the only one who read the literature. Well, another reason why Sommerfeld then had to some extent to deal with the quantum theory was because of the Solvay Congress, to which he was invited. That was organized by Nernst. So I think that apart from Einstein, who was the main promoter of the quantum theory, in Germany the people who really took it seriously were Johannes Stark and Nernst, not Planck. In fact Planck tried to get out of it. And then Nernst organized this first Solvay Congress in 1911. But the preparations were already at least half a year in advance, maybe a whole year. The members were invited, and they were asked to send in their papers something like three months in advance so that the others could all look at them. Sommerfeld wrote a paper. I don’t remember what it was, but anyway, that was one of the reasons why he had to deal with the quantum theory. But, he got really interested after the Bohr paper. And he got interested partly through me. It came about by the following. There was an old colloquium which was founded by the mineralogist Sohncke. It was held once a month in Sommerfeld's Institute. And so, it was attended by a lot of men from other departments, the mineralogists, and then people from the Technische Hochschule and so on. But Sommerfeld actually managed it and found the speakers. And so he asked me whether I couldn’t find something to speak on, and I said all right, I’ll take Bohr's paper, which just came out. And when I was preparing to give that paper I really got convinced myself, because what I tried, to do is to retrace Bohr's steps, to find the psychology of it. I wanted to follow the construction of it, to see how he proceeded. Then I was convinced myself too.
What was the reconstruction you gave?
Well, you see, from the available information the center of course was the atom of Rutherford and Geiger. Rutherford’s atom of course was available to everybody, but particularly to Bohr because he was working in Rutherford’s laboratory, and was assistant professor under Rutherford. I had known Geiger for a year, but corresponded with him on other matters, not on that. And the general impression in Germany was a nice paper, but is it correct? But of course Bohr there, having access to everything, knew that it was correct. So that that was one. The other was the circulating of quantized electrons. Now that came from Nicholson, which paper you probably know. And Nicholson got the quantization from Ehrenfest in his theory of the specific heats. Now you know what the theory of Nicholson was. He had the electrons circulating, and then he tried to calculate the Eigen-frequencies of this. And Bohr added the jumping, frequency relations. How did he get that? From two papers of Ritz. Ritz had first one on the Balmer series, rather short, that can be presented as the difference of two terms. Then he generalized it into the general combination principle. But, the question was what physical meaning have these two terms, and now from Nicholson’s paper he got that. There was the expression of the energy terms. So that he converted the difference of the terms to the difference of two energies.
Did you know of Nicholson’s work when it came out, or just much later?
No. As a matter of fact, I do not remember whether Bohr mentioned it. Probably he mentioned it, otherwise I wouldn’t have known it myself. But a couple of years later then came to Munich Fajans, a professor of chemistry. And he told me about the conditions there, how they all were struck by the lecture of Nicholson. Bohr probably also heard a lecture from him at Manchester.
That’s remarkable, because people found that old paper by 1914 or 1915, but not very many knew of it at the time, I think.
Ja, well, Bohr certainly knew because he was there. And I somehow also found it already at the time when I gave the talk. I must have given it at the end of 1913.
Did you read the Philosophical Magazine regularly?
Ja. Well, I didn’t read it from cover to cover, but we had it in Moscow, and we had it in Munich. And then what is more, I recalculated his constant by an improved method, with new data which entered. And I got it much closer actually than he got it. So that was an additional incentive.
How was your lecture received that day?
Well, those non-physicists of course received it with interest. But on Sommerfeld it must have made a very strong impression, because later, when he had himself worked on it, he note a popular report and verbatim repeated a sentence which I had said in that lecture. He must have looked in his notes.
Was mineralogy ever discussed there, at those special colloquia? I wondered if perhaps von Laue had gone to such lectures or colloquia before his discovery?
Now that's a separate subject — maybe a little later we can discuss how Laue got it. But now let me go on with this time. I was there engaged in other things, and I did not do anything myself; I mean, I became convinced but not sufficiently interested to work on the Bohr theory. Sommerfeld did get interested eventually. Not right away but a little later he put the question, “Why does the electron go just in circular orbits and not in others?” This was natural to him because he was a mathematician and knew a little about astronomy theoretically, the Kepler motion and so on. And then he generalized the relation p∅d∅ to prdr or as he told it, “Was fur das ‘∅’ recht ist, muss fur das ‘r’ recht sein.” I don’t know if you know Germans “‘r’ and ‘∅’ must enter on the same footing.”
That was the justification?
Ja. Well, I’ll tell you another thing that was discussed. This is psychological for me and does not bear on the quantum theory directly — why I was so broad-minded there, more than other physicists. And that was that I just had assimilated the theory of relativity. You see, I was one of the very restricted generation who had to work on the basis of Lorentz theory of relativity and of the electron. Then I studied for myself theoretical physics, the first thing that I worked through was the Lorentz Encyclopedia article, and then the second volume of Abraham’s book, which just came out. And that somehow was my scientific basis — the foundation of my whole education. And so when Einstein came, that was a great jolt to me. It was a complete inner reorientation of the whole, philosophy and the whole thinking. But when I was through and I accepted it, then I was completely cured of all conservatism. So after that quantum theory was small measure and I could take it in my stride.
Can you recall reactions other than those of yourself and Sommerfeld to [relativity], either at Munich or elsewhere?
One I can. Sommerfeld as I say was not a physicist and knew nothing about the work of Lorentz. But Sommerfeld was the close friend of the Gottingen relativity man [Minkowski] and believed him implicitly. So relativity was for him just duck soup. But Abraham, Abraham was one of the founders of the theory of electrons, and the Lorentz theory. He never accepted relativity.
Do you know of other reactions than those of Sommerfeld to Bohr’s theory?
Ja, well, for instance: Johannes Stark, who was so very favorable towards the theory of relativity as soon as it came, said that he was one of its promoters and founders, was against Bohr’s theory.
Do you recall why? You give an argument in one of your papers that Stark gives that the electron should know where it’s going. Was that his principal objection?
That was one of his. He just didn’t like it. You see the main thing that he didn’t like was that another man found it and not himself. Well, pardon me. The Bohr Theory he didn’t like but he did the quantum, especially the photon quantum effects, which he at once saw. But others, accepted the quantum theory, the phase integral, but not the photon effects, most of the time. Planck didn’t like either.
We were about to discuss the growth of the interest in the quantum theory of Bohr at Munich.
Ja, the Sommerfeld-Wilson theory. Wilson had it independently, although he didn’t do anything with it. He found that you get the same effect, exactly the same formula, for the elliptic as for the circular orbits, but he stopped there. Sommerfeld, being a mathematician and a relativist, investigated what happens when you do it by the theory of relativity. And he found then the splitting of the levels — the fine structure. He made a mistake there originally and integrated over an ellipse, because you can describe this relativistic motion as a rotating ellipse. And he integrated over an ellipse in both cases, which give you three times too large a fine structure. And that became clear only after my paper, that you have to integrate over the whole circle. Then you describe it as a conditionally periodic motion. You ask me my relations to Schwarzschild. Well, I did know Schwarzschild. I was introduced to him, but I was a young Privatdozent and he was a great brass, so that our relation was never intimate. But the origin of the Stark effect paper was [independent of Schwarzschild] and was as follows. Sommerfeld asked me to become Privatdozent in Munich, but the war of course queered that, as I was an enemy foreigner. So I got in touch with Zurich where I knew the professor, and they accepted. And I needed what is called a Habilitationsschrift — a thesis for becoming Privatdozent. And that was after Sommerfeld had published that fine structure paper. So I said to Sommerfeld that I would take the Stark effect which we knew because Stark himself, when he discovered it, made a tour of all the larger universities: and lectured on it. We heard about the Stark effect right after its discovery in 1913. And it is remarkable, that a week later (Wagner), who was one of the assistants of Roentgen, showed the Stark effect. He just happened to have a tube of the right kind and when the rumor of the Stark effect came before Stark announced the details, he said, “You know how he did it? He had a canal ray.” And so it worked. He built up the apparatus in a week and showed us the Stark effect, but not in hydrogen. It was mercury I think. So I started to work. Now you see I knew the Jacobi theorem because I dug it out of textbooks of mechanics, especially Appell. And then I got a copy of Jacobi’s own lecture of mechanics. It is there by a very formal method derived ... but it served, you know.
Did you search the older literature then, for instance for Stachel?
Ja, that I did later, not at that time. Now after some time again I saw Sommerfeld; it was already in the middle of the war. I had no key to the Institute any more, being an alien, but Sommerfeld was interested in my having access to literature because I wrote an encyclopedia article. He was the editor of this volume. So at one of the reunions, he told me, “I wrote Schwarzschild, that he should work on this article.” Now I was a little crestfallen, because I regarded this as a stab in the back, since he knew that I was writing it. And Schwarzschild was a mathematician of unbelievable energy; he could do everything in a twinkling. I of course couldn’t reproach him, but I decided, “Now I have no prospects unless Schwarzschild should go to heaven.” And the next day when I was going to bed, I had the idea of the limit. You see, I knew already how the electron moves, and I knew now to do it. I got up at 5 o’clock the next morning and by 10 I had the formula. And then the same morning I brought it to Sommerfeld. And what do you know, the same afternoon he got a letter from Schwarzschild, and Schwarzschild had the wrong formula. It was the same order of magnitude, but didn’t agree, on the positions of the lines. Sommerfeld wrote Schwarzschild, “This morning Epstein brought me the formula of the Stark effect, and this afternoon we got your letter. But Epstein’s formula agrees with the observation.” When Schwarzschild got [his result], he immediately announced in the Berlin Academy that he would speak about it. And he did, before he wrote the letter to us, so he reported it wrong in the Academy. By that time however I had already sent my announcement that came out just one day before he delivered that lecture in the Academy. But in the lecture he corrected it, and in the proof he removed all the discrepancies, and it came out correct also. Of course the two final papers came out much later. So I had the priority by one day.
Did you get the separation coordinates from Jacobi’s earlier work?
Well, it is possible that Appell has it, but I doubt it. I think Appell has a case of two centers, not of one. It happened that my thesis was on the diffraction from parabolic cylinders and parabolic coordinates were old hat to me. The trick was to find the limits. And that time I had a good thing, but with the advent of quantum dynamics of course it was erased.
Was the Stark effect considered to be a quantum problem par excellence? That is, in distinction from the Zeeman effect, which had been solved classically.
The Zeeman effect you say. Once Sommerfeld, in a course of lectures centered on Kepler motion and its applications, which he gave during the war explained one of his big papers to us. There were just a handful of students during the war. And then he came to the Zeeman effect. At that time I actually worked out the Zeeman effect and gave it to him, so that what he wrote in that Zeeman effect paper was my work. He is not quite to blame that he didn’t say so, because he states in one place, “Mit Epstein” — “Following Epstein, we assume that —.” He thought that I had sent the paper to be published, but I hadn’t. Well, there was still the lacuna that we didn’t know the selection principle. That was found by Rubinowicz, as you probably know. You see, in the Stark effect, by a coincidence, this doesn’t make any difference. Of course the third quantum, the azimuthal, only changes by one or zero, but it happens that if you change it by one and two and three you don’t get a new line.
In a paper by Warburg that was published in 1913, which you refer to, he says that the Stark effect is really the one in which the quantum is obviously involved, whereas the Zeeman effect can be understood classically. Was it considered to be next problem that the Bohr atom should have to solve?
I at least considered it that way.
Had you done any earlier work on the quantum at all?
No, that was the first paper. In the preceding years I still worked on Maxwell’s theory. There was of course my thesis. On the relativistic topic I had a paper in 1911, that is the Russian paper. Then the ponderomotive effect of light, that was 1914. Then light pressure under Sommerfeld, that was theory of Maxwell, and then the thesis. This does not quite complete that calculation of mine, but the main things are here. Then there is a paper on thermodynamics, and that encyclopedia article. And then comes the theory of the Stark effect. … So that is the source of quantum theory and that is all I know about the quantum thing. Well, what other questions do you have to talk about?
You said you would tell me something about von Laue.
Ja. Von Laue — it doesn’t belong here, but it is interesting. You see, there was a student of Sommerfeld of the name of Ewald, who is now in this country, and his thesis, which used … a cubical lattice as a model for the theory of dispersion. And Ewald was practically finished, but he had some question of principle of which he was not quite certain. Sommerfeld couldn’t help him there, and he thought perhaps Laue could, because Laue had worked on optics and the theory of diffraction, linear lattices, and so on. So he went to him at his home and explained it to him. And that was when Laue first so to say heard about the Sohncke theory of lattices. He was a member of the Sohncke colloquium for years, but he somehow missed who Sohncke was. I got the book of Sohncke and learned the Sohncke theory of crystals. But Laue then didn’t know it, and didn’t know that crystals were lattices. But when he had that, he asked Ewald what the lattice distance was, and Ewald said it was 5.10-8. At that time Sommerfeld had gotten the old plates of Haage and Wind who, in 1896 had tried to measure the [grating space for] diffraction of X-rays, and by micro-photometry, which a man in Munich did, he found out that probably it was of the order of 10 to the minus eight. And so Laue was already musing how to find a diffraction lattice for X-rays, and suddenly a bell rang. And the very next day he came to the Cafe and told me what it was he wanted to measure. And I said it’s a good thing.
Of course I did not in that moment understand the whole importance of it, but I was a little helpful to Laue too in that when he brought this idea to Sommerfeld and Debye — I think Debye just happened to be around although he was actually at that time teaching in Zurich — they told him that it wouldn’t work because of the thermal motion. And Sommerfeld, just on the strength of Debye probably, pooh-poohed it, you know. But when he came to talk to me, I pointed out to him that if it were a momentary photograph then they would be right, but here it oscillates about a position of rest. The photographic plates only slowly integrate it, so that the thermal motions will cancel out. But somehow Sommerfeld didn’t accept this even yet, and actually Laue did it by himself and Sommerfeld wasn’t there. By that time Sommerfeld had engaged as assistant Friedrich, to do some X-ray experiments which he was interested in himself. But he went off on this vacation and Friedrich was hanging around without anything to do, so Laue told him what to do.
What did Sommerfeld say when he heard of this?
He was rather disconcerted, but he was an honest fellow and admitted that it was a big thing. And from then on he promoted it.
It’s very amusing, because Debye in 1912 and 1913 published the papers on the thermal effect…
because he had to explain it to himself!
Do you recall the paper that Bohr wrote on the Stark effect before your own? You mention it also.
I don’t know that paper. Could you recall it to me?
It’s a very funny paper. He begins by saying that the motion in general is very complicated, but that there are stationary states, and these come out to be oscillations through the nucleus, where the electric field just displaces the center of oscillation. It’s sort of a Thomson atom even, where the electron is quasi-elastically bound inside the charge and the electric field displaces it. And it’s with those orbits that he calculates, only those. It’s a very peculiar paper. Perhaps you’d be interested in looking at it later.
Ja, well, you will come back tomorrow. I could look at it tonight maybe… Now, what is next?
Well, there are the questions about your other work in 1916.
Ja, well having found what I thought was a general rule, I considered it my duty to apply it to as many things as I could. This work on the quantization of the electron was not quite right — the hypothesis that the parabolic motions are also quantized was not correctly applied. It was a somewhat dragged-in assumption at that time. But I got it straight a couple of years later in the Leiden paper. Of course it comes out very much simpler in wave mechanics. Years after the Leiden paper came out with correction, Nicholson wrote that this paper of 1916 was incorrect. Well it wasn’t incorrect but it was strange. So I wrote him back that he should have looked at the other.
What was the reception of that paper in general? Did many people think that that was the way to attack the beta decay and the photo-effect?
Well, some people did. You see by that time I didn’t believe it myself already. (Konigsberger) in Freiburg took it quite seriously, but I wrote him that I was not quite sure of it.
You make a very interesting remark in that paper, where you have got the energies that you expect to observe in the photo-electric effect. You say that Sommerfeld doesn’t take the Bohr frequency condition as meaning very much energetically.
Ja, he didn’t. If I understand what is meant, it is a reference to quantum, that it is just a rule for getting //the electron energies we want.// I have a paper in the Bavarian Academy about January 1919, where I also treat it as if there were no quanta. Some things may be right, but many things are wrong. I wanted to get it clear in my own mind. That is the reason why I worked so much on it. Incidentally, at that time I had private difficulties. And especially in Zurich I couldn’t devote so much time to original work. I had all sorts of other things on my hands. You put there a question with respect to the Bohr model of the hydrogen molecule, which incidentally Debye tried to appropriate. In the beginning I took it quite seriously. My first reason for doubting it was actually that the analogous helium atom didn’t work. I can’t remember when which measurements came out, but when they did //they were conclusive.// And I worked quite a bit in addition to this Kreisel paper //on other models//.
Have any of those survived? Do you have any of those manuscripts?
All my papers at Munich, my whole library and everything were burned in a fire there. And I worked on a model that there is an inner electron close by [the nucleus]. Then there was the work of van Leeuwen and Lorentz, who showed that the Bohr electron was unstable. Now that I did not take as completely convincing, because then nobody knows what the quantum conditions did. … I think now that it is an important paper, but at that time there was no basis for judging. But the main thing is that the analogous helium effect didn’t work, so I was doubtful. And for that reason I also thought that there probably was an inner electron… but that is neither here nor there.
In 1916 you wrote an interesting paper — on specific heats.
I did several evaluations and one is correct. And that did not agree with the measurements. [Neither did Ehrenfest’s but] of course that Ehrenfest’s doesn’t agree is not surprising, because it was one-dimensional. The two-dimensional //is better, but the absolute values don’t agree.//
How closely was the problem of specific heats considered to be bound up with the whole Bohr atom?
You see the problem of specific heats was older than the Bohr theory. It arose from Ehrenfest’s paper, on the rotational specific heats; Eucken's measurements confirmed it experimentally. So it had no connection with Bohr's atom except that Bohr used the Ehrenfest [quantization] rule.
Yes, I was just wondering what effect the fact that it is difficult to get agreement with the measured specific heats using the Bohr molecule, or the Bohr-Debye model, or the symmetric top or the asymmetric top, had on people’s willingness to consider the Bohr atom completely successful.
Well, I can’t tell you how generally. I did not take it very seriously after that. On me it had an effect. But, for instance Debye took it quite seriously. Because Scherrer was a student at that time who wrote his doctor’s thesis on that model. He calculated diffraction and rotation of polarization and so on. I didn’t think much of it because it was mathematically a poor paper.